:: Ord a => [a] -> [a] -package:html-entities -package:ghc -package:mono-traversable -package:basement -is:exact -package:statistics package:witherable -is:exact is:exact

Removes duplicate elements from a list, keeping only the first occurrence. This is asymptotically faster than using nub from Data.List.
>>> ordNub [3,2,1,3,2,1]
[3,2,1]